Please ensure that your plugins are updated (check the manufactirer's website to ensure)
Make sure that images you are trying are not missing in Lightroom.
If the steps mentioned above do not help, Reinstall the problematic plugin.
Just installed Lightroom cc for the 1st time. The plugins for Aperture/iPhoto import and all the ON1 plugins are greyed out.
Do you mean that the menu command File > Plug-in Extras > Import From Aperture Library is greyed out?
(Note that the names of the plugins in the menu, e.g. "Aperture/iPhoto Importer Plug-in", will always be greyed out.)
If your catalog is brand new and empty, try exiting and restarting LR. There is a minor bug where many plugin menu commands will be greyed out on a brand new catalog, but restarting LR fixes the issue.
With the ON1 plugins, I think you'll need at least one photo to be selected when you invoke them.
You won't be able to reinstall the Aperture/iPhoto Importer Plug-in, since that comes bundled with LR.
Thanks. Restarting solved the problem.